1. Field of the Invention
The present invention relates to a composition for forming a film for protecting wiring made of Cu or the like and formed on a printed wiring board, in particular, a suspension substrate for hard discs. The present invention also relates to a dry film for forming a wiring-protecting film, using the composition, and a substrate having wiring protected by means of the dry film.
2. Description of Related Art
In order to record or reproduce information in a hard disc device used as a memory device of a computer or the like, a magnetic head is relatively traveled on a magnetic disc inside the device. The magnetic head and the magnetic disc are designed to keep a very small, constant interval therebetween against air flow generated by the traveling. A magnetic head supporting device for pressing the magnetic head onto the magnetic disc elastically against the air flow in this way is a suspension. In recent years, as the suspension, there has been used a member wherein a circuit is formed directly on a stainless steel plate, several ten micrometers in thickness, having spring performance. In the step of fabricating a hard disc, such a suspension is set on a magnetic disc. This is performed using a special tool, a robot arm, or the like. At this time, it is feared that gold plating on wiring laid on the suspension substrate is injured. In order to remove this fear, a wiring-protecting layer for protecting the wiring portions is usually laid. Hitherto, liquid photosensitive polyimide has mainly been used in this wiring-protecting layer of the suspension. This material is chiefly applied by spin coating, and then dried, exposed to light, developed and cured, thereby forming a protecting layer. However, it is difficult to perform the spin coating after wiring is formed on the suspension substrate or the external form of the suspension substrate is worked. It is also difficult to collect and recycle the solution which has not been used to form the film. As described herein, the spin coating has various problems. The photosensitive polyimide also has problems that: the polyimide is not easily made into a thick film since the polyimide has a poor light transmissivity; development using an organic solvent is required and the working environment for the development is undesirable; and a great amount of environment polluting material is discharged. In order to solve these problems, a protecting film material in a dry film form is suggested in Japanese Patent Application Laid-Open (JP-A) No. 10-289432. In this method, however, it is unavoidable to carry out development using a high-concentration alkali solution in water. Moreover, when this protecting film material is used in a thin suspension substrate, the suspension substrate receives shrinkage stress at the time of the formation of a protecting film, so as to warp up. Consequently, a problem that inconvenience is caused at the time of other workings is encountered.
In recent years, the following has become a problem: when parts such as semiconductor elements are mounted on plated circuits as performed about a suspension substrate of a hard disc driver, the breakdown of the elements on the basis of static electricity produces a bad effect on the yield thereof. This is caused on the matter that static electricity is given to the insulated layer which constitutes the substrate. In order to overcome this problem, the insulated layer is subjected to static elimination. As a method for the static elimination, suggested are, for example, a method of spraying an antistatic or electroconductive paint, which contains an antistatic or electroconductive substance, onto the insulated layer so as to form an antistatic layer, as described in JP-A-11-286082; or a method of using, in the insulated layer, a resin having a low surface resistance of about 106 to 1012Ω. However, according to the method of forming the antistatic layer in the spraying manner, there arises a problem that an excessive step is required, examples of which include the step of removing unnecessary portions of the antistatic layer or the step of protecting areas where no antistatic layer is necessary with a resist before the spraying, and removing the resist after the antistatic layer is formed, in order to prevent the antistatic layer from being formed on the areas where no antistatic layer is necessary. The spraying method also has a problem that the thickness of the antistatic layer is not easily controlled so as to become uneven. The method using the low surface resistance resin in the insulated protecting layer has a problem that the resin cannot attain an insulation level required for wiring having a high density and sufficient insulation reliability cannot be ensured at the present time, when a high wiring density is desired.